If you want to write an orchestral composition, you can start with a sketch, perhaps nothing more than a melody. Or you can take an existing composition and arrange if for symphony orchestra. Both methods are being used, but starting out with a sketch is more difficult, because you might end up writing parts straight into the orchestral score. I wouldn't advise that to a inexperienced composer.
Even if you use an existing composition, you need to understand the harmony. Preferable classical harmony (with the Roman Numerals). That way you'll be able to extend the parts, maybe even write some melodic lines. But if you only know chords, a good understanding of keys can be sufficient.
In this section you'll find some examples of orchestrations. You can download the mp3, the orchestral score and the original score or sketch for comparison. It's free.
The original of 'Sad Old Witch' is a composition for piano. In the download you'll find an mp3 and a score of the piano version as well. Have a look at the beginning. How would you orchestrate an arpeggio?
'The Happy Pig' is an easy piece for piano from the album The Calendar. The orchestration is kept simple, although there are some changes in the repeat. The original piano piece is suited for children.
The next composition is also based on a piano piece, but now I took the liberty to add several melodic lines. Compare the two scores and you will notice the difference.
'Big Bad Wolf' is based on a mere sketch. All I had was a melodic line, but that is usually sufficient, because even a solo melody is part of a harmony.
to be continued...